(1) Field of the invention
The present invention relates to a system and method for controlling fuel injection for an internal combustion engine mounted in a vehicle and particularly relates to the system and method for controlling fuel injection for the internal combustion engine which provide a flat characteristic for an air-fuel mixture ratio even at the time of an engine transient operating condition.
(2) Background of the art
It is necessary to control fuel supply quantity with good responsive characteristic according to a degree of engine output power requirement when an output power required for a vehicular engine is changed. The result of the fuel supply quantity control affects an air-fuel mixture ratio particularly during an engine transient operating condition and affects engine operating performance such as a drive feeling and exhaust gas composition.
In general, a deviation of the air-fuel mixture ratio from a target air-fuel mixture ratio during an acceleration/deceleration of the engine is caused by a change in quantity of fuel adhered or floating onto or around a wall surface of intake manifold or intake port of an intake air passage. Such fuel quantities adhered and left on or around the wall surface are largely changed according to the engine operating condition.
A Japanese Patent Application First (non-examined) Publication Showa 58-104335 published on June 21, 1983 exemplifies a previously proposed fuel injection quantity controlling system for an internal combustion engine.
In the above-mentioned Japanese Patent Application First Publication, the fuel injection quantity controlling system derives a required load from an output of an airflow meter installed on an upstream of an engine throttle valve and calculates fuel injection quantity from the required engine load.
In addition, during the engine transient state (,e.g., acceleration), the fuel injection quantity is corrected by a transient state correction quantity, with quantities of fuel flowing toward a wall surface of a passage of an intake manifold and/or of an intake port (so-called, wall current) taken into account. It is noted that the transient state correction quantity is a correction quantity to correct the fuel injection quantity and derived for compensating for the quantity of fuel flowing toward the wall surface of the intake manifold and/or intake air port (wall current).
The transient state correction quantity is not only used during the engine acceleration and deceleration but also used for the correction of fuel injection quantity upon a fuel recovery immediately after a fuel cut-off or for that upon an engine start. Such transient operating conditions described above will have a great influence on the quantity of the wall current.
However, in the previously proposed fuel injection quantity controlling system and method, the quantity of fuel flowing toward the wall surface which provides a basis for calculating the transient state correction quantity is derived only on a basis of an output of an engine coolant temperature sensor (,i.e., engine coolant temperature). Therefore, a sufficient correction of fuel quantity at the time of such transient operating conditions as described above cannot be carried out by the transient state correction quantity.
That is to say, although a temperature at the wall surface of such a fuel-adhered portion of the intake air passage is an essential parameter to determine vaporization of the wall current, the temperature at the fuel-adhered portion is largely varied for different engine operating conditions, as shown in a characteristic graph of FIG. 6. In addition, delay time constants of the temperature increase and/or decrease at the wall surface are provided for different engine operating conditions, as shown in a characteristic graph of FIG. 7.
In other words, since a gradient of increasing the temperature at the fuel-adhered portion becomes different depending on the engine operating condition, this appears as the difference of the delay time constant. Hence, since the temperature of the wall surface at the fuel-adhered portion becomes different depending on the engine operating condition before the acceleration, the engine driveability becomes worsened due to an inappropriate air-fuel mixture ratio during the transient operating condition such as acceleration and/or deceleration.
Particularly, in the engine of a multi-point injection type in which a quantity of fuel is directly injected toward an intake valve 32 in a cylinder head 31 by means of a fuel injection valve (fuel injector) A, as shown in FIG. 10, and in which a quantity of fuel is injected toward a wall surface of the intake port by means of a fuel injector B, as shown in FIG. 10, change patterns of the air-fuel mixture ratios after the engine has started in both types of the multi-point injections are different from each other and the change of the air-fuel mixture ratio in the case where the fuel is directly injected toward the intake valve 32 is more remarkable.
That is to say, an error of an air-fuel mixture ratio due to a temperature at the fuel-adhered portion, i.e., the intake valve 32 is large and such a phenomenon as described above does not only occur at a time immediately after the engine has started but also tends to occur, e.g., at a time immediately after the fuel supply is resumed after the fuel cut-off.
Hence, during the acceleration, a, so-called, hesitation easily occurs. In addition, a deviation of the air-fuel mixture ratio from a three (ternary) -catalytic point in a CCRO (catalytic converter rthodium) or CCO (catalytic converter oxidation) occurs due to the variation of the air-fuel mixture ratio so that the exhaust gas clarification characteristic of the engine is reduced.
It is noted that although a sensor, e.g., for detecting a temperature on the fuel-adhered portion (wall surface) may be installed on the wall surface or other position to detect the temperature at the fuel-adhered portion, this results in the increased cost of mounting such a sensor and this introduces poor productivity of installing the fuel injection controlling system in the engine.
A Japanese Patent Application First (non-examined) Publication No. Showa 59-101556 published on June 12, 1984 exemplifies another previously proposed fuel injection controlling system for the internal combustion engine.
In the other previously proposed fuel injection controlling system disclosed in JP-Al-59-101556, a basic fuel injection quantity T.sub.p is calculated on the basis of an intake air quantity Q.sub.a and engine revolutional speed N. In addition, an opening angle of an engine throttle valve is detected. When the opening angle of the throttle valve increases at a rate exceeding a constant valve, a temporary addition of fuel (interrupt fuel injection) is immediately executed independently of the basic fuel injection quantity so that an incremented correction of fuel quantity is executed according to a degree of acceleration and an insufficient quantity of the basic fuel injection quantity T.sub.p due to a response delay of measured intake air quantity at the time of abrupt acceleration is compensated for by means of the interrupt fuel injection determined according to the angular displacement of the throttle valve.
However, in the above-described other fuel injection quantity controlling system disclosed in JP-Al-59-101556, the quantity of fuel injected during a suction stroke or immediately after the start of the suction stroke is hardly sucked into the corresponding cylinder (combustion chamber) during its suction stroke and largely sucked into the corresponding cylinder during the subsequent suction stroke. Therefore, the fuel injection quantity controlling system cannot take a full advantage of an asynchronization (interrupt) fuel injection described above. Even if a pulsewidth of an injection signal supplied to the fuel injector is widened, the air-fuel mixture ratio appearing at the second suction stroke becomes too rich.
A Japanese Patent Application First (non-examined) Publication Showa 64-3245 published on Jan. 9, 1989 exemplifies an air-fuel mixture ratio controlling system which comprises: (a) first means for determining whether the asynchronization fuel injection should be carried out independently of the synchronization fuel injection for each rotation of the engine on the basis of a quantity of change in a signal representing an engine operating condition; (b) second means for calculating an asynchronization fuel injection quantity for each cylinder on the basis of the quantity of change of the engine operating condition signal when the asynchronization fuel injection is carried out; (c) third means for calculating a percentage of correction of the asynchronization fuel injection quantity for each cylinder according to a cycle position signal at that time when the asynchronization fuel injection should be carried out; (d) fourth means for correcting the asynchronization injection quantity from the calculated correction percentage; and (e) fifth means for actuating the fuel injection valve for each cylinder in response to a drive signal which corresponds to the corrected asynchronization fuel injection quantity.
The above-described air-fuel mixture ratio controlling system disclosed in JP-Al-64-3245 can considerably eliminate deviations of the air-fuel mixture ratio from a target air-fuel mixture ratio for respective cylinders with differences of wait time intervals from the occurrence of the asynchronization fuel injection to the entrance of the suction stroke immediate after the asynchronization fuel injection taken into account.
In addition, the engine driveability can be assured and exhaust gas clarification characteristic with misfire and torque reduction can be improved.
Although the above-described air-fuel mixture ratio controlling system disclosed in JP-Al-64-3245 supplies a smaller amount of fuel injected at the time of the subsequent synchronization fuel injection so as to compensate for an extra quantity of fuel injected at the time of asynchronization fuel injection in order to prevent richer air-fuel mixture ratio, it was confirmed that the richer and leaner the air-fuel mixture ratios were generated individually for the respective cylinders, a, so-called, glitch appeared in the emission characteristic (the whole air-fuel mixture ratio indicated no flat characteristic), and an improvement in a catalytic effect of the three catalytic converter (CCRO or CCO) was needed.
Furthermore, a Japanese Patent Application First (non-examined) Publication No. Showa 58-8238 published on Jan. 18, 1983 exemplifies a previously proposed fuel injection quantity controlling method for a fuel-injection type engine.
In the above-described proposed fuel injection quantity controlling method disclosed in JP-Al-58-8238, a quantity of fuel adhered to the wall surface is estimated and calculated according to the quantity of fuel injected through the fuel injection valve, assuming that the quantity of fuel adhered to the wall surface and the quantity of fuel brought away from the wall surface into a combustion chamber of the corresponding cylinder during the suction stroke are changed according to the quantity of injected fuel.
Then, the calculated quantity of fuel adhered onto the wall surface is accumulated to derive the quantity of fuel brought away from the wall surface into the combustion chamber. Then, the quantity of fuel brought into the combustion chamber is subtracted from the calculated quantity of fuel adhered onto the wall surface and is added to the synchronization fuel injection quantity to derive an actually executed fuel injection quantity. That is to say, when the quantity of fuel adhered onto the wall surface is large, the synchronization quantity of fuel is increased and when the quantity of fuel brought into the combustion chamber from the wall surface is great, the synchronization fuel injection quantity is reduced to suppress the variation of air-fuel mixture ratio.
In the above-described previously proposed fuel injection quantity controlling system disclosed in the JP-Al-58-8238, the fuel injection correction quantity is corrected for a behavior of the wall current which is changed with a relatively slow time constant (so-called, a low-frequency wall current component).
However, since no correction is carried out for the behavior of the wall current which is changed with a relatively high-speed time constant (so-called, high-frequency wall current component), the following problem occurs.
That is to say, during a slow acceleration such that no interrupt injection (asynchronization fuel injection) is needed, the air-fuel mixture ratio (A/F) at the time of a first suction stroke in which the acceleration is started becomes slightly lean. This is because the rate of the incremented quantity of fuel is large which has directed as the wall current when the timing at which the synchronization fuel injection occurs immediately before the suction stroke.
In addition, when the injection timing is too early with respect to the suction stroke, the air-fuel mixture ratio becomes lean due to the injection carried out in response to an old air quantity indicative signal derived from the airflow meter.